The present invention is directed to voltage reference circuits, and in particular to bandgap voltage reference circuits for use with emitter coupled logic (ECL) and analog circuits.
Most ECL and analog logic gates require an appropriate voltage reference for proper operation. For example, some of these types of circuits require a voltage supply that must be substantially temperature-independent. One type of reference circuit that is typically employed to provide an appropriate voltage level is referred to as a bandgap voltage reference circuit. This circuit is so named because it provides an output voltage that is approximately equal to the bandgap voltage of silicon.
To facilitate an understanding of the objectives of the present invention, the details of a conventional bandgap voltage reference circuit will first be described. Referring to FIG. 1, a typical reference circuit includes a transistor 10 having an emitter connected to a supply voltage VEE by means of a resistor 12. By way of example, the supply voltage VEE might have a nominal potential of about -4.5 volts relative to a ground potential VCC. The collector of the transistor 10 is connected to the ground potential by means of a resistor 14 and the collector-emitter path of a transistor 16.
A diode connected transistor 18 has its common collector/base connected to the base of the transistor 10 by means of a resistor 20. The emitter of the transistor 18 is directly connected to the supply voltage VEE, and its collector/base is also connected to the ground potential VCC by means of a resistor 22 and a transistor 24.
Another transistor 26 also has its emitter directly coupled to the supply voltage VEE and its collector connected to the ground potential VCC by means of a voltage divider comprising resistors 28 and 30. The base of the transistor 26 is connected to the collector of the transistor 10. The bases of the transistors 16 and 24 are connected to the junction of the resistors 28 and 30 in the voltage divider. A compensation capacitor 31 is connected between the base and collector of the transistor 26 to provide stable operation.
In operation, the transistors 10, 18 and the resistors 12, 22 form a logarithmic current source in which the current density in the emitter of the transistor 10 is less than that of the transistor 18 because of the voltage developed across the resistor 12. The temperature variation of the collector current in the transistor 10 can be suitably adjusted through proper selection of the values for the resistors 12 and 22. The transistor 26 senses the temperature-dependent voltage that is developed across the resistor 14 and controls the current through the voltage divider 28, 30. The divided voltage developed across the resistors 28 and 30 is applied to the bases of the transistors 16 and 24. A temperature compensated output voltage VCS is produced at the emitter of the transistor 24.
The output voltage VCS is greater than the supply voltage VEE by an amount equal to the base emitter voltage of the transistor 26 (V.sub.BE26) plus the voltage across the resistor 14 (V.sub.R14). Under ideal conditions, any change in the supply voltage VEE should result in a corresponding change in the output voltage VCS. In other words, the value (VEE-VCS) should always remain constant. In practice, however, this condition does not occur with the circuit shown in FIG. 1.
For example, if the supply voltage VEE becomes more negative, to increase the absolute value of VEE-VCC, this increase in voltage develops across the resistor 30, causing an increase in current through this resistor. This condition causes a corresponding increase in the collector current of the transistor 26, resulting in an increase in its base-emitter voltage (V.sub.BE26) Since the output voltage VCS is dependent upon V.sub.BE26, the difference between the supply voltage VEE and the output voltage VCS will not remain constant. For example, at room temperature the ratio of the change in VCS to the change in VEE might be around 0.98. Ideally, this ratio should be 1.
To overcome this problem, the collector current in the transistor 26 must be maintained constant. In the past, one approach towards maintaining a constant collector current has been to substitute a pnp transistor current source for the resistor 30. The pnp transistor conducts in inverse proportion to the supply voltage changes, to thereby maintain a constant current through the collector of the transistor 26.
Alternatively, it has been proposed to place a pnp transistor in shunt across the resistor 28, to keep the current through this resistor constant.
These approaches which employ pnp transistors to maintain a constant current through the collector of the transistor 26 are not well suited for use in ECL circuits. More particularly, conventional ECL fabrication techniques are optimized for the production of good npn transistors, and result in the production of relatively poor quality pnp transistors. Typically, a pnp transistor produced by a conventional ECL process has a gain of 1 or less. Thus, the reliability of the pnp constant current source becomes process dependent in ECL circuits. It is desirable to avoid this drawback associated with previous approaches to providing a constant difference between the supply and output voltages.